A wireless data link provides a reliable, robust, and efficient means of transporting blocks of data from a mobile or handheld data processing workstation to a header or base station. The base station may be attached to a wired Local Area Network (LAN), such as an Ethernet network, and forms a connection into the LAN. The mobile workstation may employ standard, high-level network protocols, such as TCP/IP, to access the LAN. From the point of view of an operating system and application, programs running on the workstation transport over the wireless link occurs transparently.
Such a mobile wireless link, particularly one that employs infrared (IR) light as a communication medium, presents a communications reliability problem that is distinct from the problem of data transmission errors occurring at the bit level. As the mobile workstation is moved, or as optically opaque objects in the environment of the workstation move, the reception of optical signals transmitted between the mobile unit and the one or more base stations may be interrupted, strongly reduced by "shadowing" or corrupted by multi-path effects. Such an optical wireless link cannot therefore be treated as a reliable medium and specific provisions must be made for dealing with the inherent unreliability.
It is particularly desirable to avoid deadlock situations such as might arise when a connection is lost during a transaction. These can be resolved by timeout mechanisms, but this technique becomes burdensome if disconnection events occur frequently.
Problems related to signal processing and clock and bit recovery become progressively worse as the bandwidth of the wireless link is increased. This is due at least in part to admitting more noise into the receiver as a band-limiting filter is made wider and is also due to a need to compensate for inter-symbol interference as the data bit width approaches the rms delay spread of the multiple paths.
In IBM Technical Disclosure Bulletin, Vol. 20, No. 7, December 1977 F. Closs et al. describe the use of diffuse transmission of infrared signals for wireless communications between a controller and a plurality of terminals. Indirect links rely on infrared radiation that is diffusely scattered from walls and ceilings- The use of different wavelengths or different carrier frequencies is disclosed for separating channels.
In IBM Technical Disclosure Bulletin, Vol. 24, No. 8, page 4043, January 1982 F. Gfeller describe general control principles of an infrared wireless network incorporating multiple ceiling mounted transponders that couple a host/controller to multiple terminal stations. A downlink infrared channel operates at 200 kHz and an uplink infrared channel operates at 400 kHz. Access to the uplink channel is controlled by a Carrier Sense Multiple Access/Collision Detection (CSMA/CD) method.
In commonly assigned U.S. Pat. No. 4,402,090, issued Aug. 30, 1983, F. Gfeller et al. describe an infrared communication system that operates between a plurality of satellite stations and a plurality of terminal stations. A host computer communicates with the terminal stations via a cluster controller and the satellite stations, which may be ceiling mounted. Communication with the terminal stations is not interrupted even during movement of the terminal stations. In a disclosed embodiment a carrier frequency for the infrared link is 100 kHz and a data speed is 50 k Bit/s. Wired communication between the satellite and the cluster controller occurs at 1M Bit/s.
What is not taught by the prior art is the provision of a robust control channel that is separate from a data channel. The utility of such a separate control channel becomes apparent from propagation studies of infrared links operating at high bandwidths (&gt;10M Bits/s). These studies indicate that it may be difficult to sustain a continuously reliable link at such high bandwidths. Thus, if a control dialogue is also communicated over such a high-speed link frequent disconnections of the mobile workstation from the network can be expected to occur, resulting in an excessive overhead due to re-establishing the connection.
However, relatively low bandwidth links (&lt;50 k Bits/s) have been found to be extremely robust. That is, diffuse infrared propagation renders low bandwidth links less susceptible to loss of data.
It is therefore an object of the invention to provide a reliable and efficient infrared data communications network.
It is another object of the invention to provide an optical communication system that includes a separate control channel that has a lower bandwidth than a data channel.
It is a further object of the invention to provide an infrared communication system with a diffuse propagation channel for implementing a low-bandwidth control channel that is separate from a higher-bandwidth data transmission channel.